SOLID-STATE NUCLEAR MAGNETIC RESONANCE (ssNMR) METHOD FOR DETECTING HYDROGEN BOND STRUCTURE

ABSTRACT

An experimental technology for detecting a hydrogen bond based on an ssNMR technology includes: (1) exciting a 1H nucleus of an RNA sample with a π/2 pulse; (2) applying two π pulses every half rotation period on an X_nucleus of the RNA sample; (3) applying a π pulse on the 1H nucleus of the RNA sample; (4) applying two π pulses every half rotation period on the X nucleus of the RNA sample; (5) applying a 90° pulse on 1H and X atoms of the RNA sample; (6) recording a chemical shift of X in indirect dimension; (7) applying the 90° pulse on the 1H and X nuclei of the RNA sample; (8) repeating steps 2, 3, and 4; and (9) collecting the 1H signal in direct dimension; where X is selected from the group consisting of 15N and 13C.

TECHNICAL FIELD

The present disclosure relates to the field of solid-state nuclear magnetic resonance (ssNMR) spectroscopy, in particular to an experimental technology for detecting a hydrogen bond based on an ssNMR technology.

BACKGROUND ART

A hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative “donor” atom or group, and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor. The hydrogen bond is generally denoted X-H···Y, where X and Y can be (N), oxygen (O), and fluorine (F) atoms. The hydrogen bond plays an important role in molecular interactions and affects the structure, function and dynamic characteristics of substances. For example, in supramolecular chemistry, hydrogen bonds can precisely control and guide molecular structures; in biological macromolecules, such as ribonucleic acids and proteins, the hydrogen bonds are the basic building block of the structures. Through the hydrogen bonds, complex secondary and tertiary structures can be formed in biological macromolecules to exert physiological functions. The hydrogen bond has attracted extensive interest of researchers since its discovery. Direct observation of hydrogen bonds and measurements of hydrogen bond strengths are important research topics in many disciplines, such as materials science, chemistry (inorganic and organic chemistry, supramolecular chemistry, and biochemistry), molecular medicine, and pharmacy. Therefore, it is crucial to detect the hydrogen bond.

The current research methods of detecting the hydrogen bonds are classified into X-ray crystal diffraction, neutron diffraction, infrared spectroscopy, and nuclear magnetic resonance. In the X-ray crystal diffraction, the samples should form a regular single crystal. Therefore, the X-ray crystal diffraction is not suitable for samples that are difficult to be crystallized. In addition, the X-ray crystal diffraction does not allow direct observation of hydrogen atoms. The neutron diffraction can help detect molecular vibration, but is not widely used currently. The infrared spectroscopy can be used to study a hydrogen bonding mode, but cannot provide site-specific information. Nuclear magnetic resonance can provide rich structural information and has a wide range of applications, but still faces difficulties such as lack of universal detection methods to observe the hydrogen bonds. Therefore, in the present disclosure, a solid-state nuclear magnetic resonance (ssNMR) method is proposed for the specific detection of N—H...N and N—H...O═C hydrogen bonds.

The ssNMR can provide structural information of solid samples in an atomic scale, is applicable to characterize samples in diverse forms, has no limitation on molecular weight, and features non-destructive detection. The ssNMR has outstanding advantages in the study of protein-protein and protein-nucleic acid complexes with low solubility and/or undergoing biological phase transitions. At present, there are several ssNMR techniques for hydrogen bond structure analysis, including two-dimensional ¹⁵N-¹⁵N RFDR experiments and two-dimensional proton-detected ¹H-¹⁵N hCN(PAR)NH experiments. The two-dimensional RFDR experiments indirectly prove the N—H...N hydrogen bond by collecting a ¹⁵N-¹⁵N two-dimensional spectrum to obtain correlation of the ¹⁵N atoms close to each other, but cannot directly prove existence of the hydrogen bond. Moreover, the method uses ¹⁵N detection with a low sensitivity. The two-dimensional proton-detected¹H-¹⁵NhCN(PAR)NH experiments realize the transfer of ¹⁵N-¹⁵N using a proton-assisted recoupling (PAR) mechanism. The detection of N—H...N hydrogen bond can be achieved by optimizing parameters based on the proposed hydrogen bond structure, such as the length and the angle of the N—H...N hydrogen bond. However, it is difficult to optimize the transfer efficiency of the PAR experiments, making it difficult to apply the two-dimensional proton-detected ¹H-¹⁵N hCN(PAR)NH experiments into the hydrogen bond observation. Therefore, existing ssNMR methods that can detect the hydrogen bond still face technical bottlenecks such as low accuracy, poor sensitivity, and difficult experimental setup. Based on this, the present disclosure aims to develop an ssNMR method with a high sensitivity, an excellent accuracy, and simple setup for specific detection of the hydrogen bond.

SUMMARY

In view of this, the present disclosure provides an ssNMR method for detecting a hydrogen bond structure, including the following steps: (1) exciting a ¹H nucleus of an RNA sample with a π/2 pulse; (2) applying two π pulses every half rotation period on an X nucleus of the RNA sample; (3) applying a π pulse on the ¹H nucleus of the RNA sample; (4) applying two π pulses every half rotation period on the X nucleus of the RNA sample; (6) applying a 90° pulse on ¹H and X atoms of the RNA sample; (7) recording a chemical shift of X in indirect dimension; (8) applying the 90° pulse on the ¹H and X nuclei of the RNA sample; (9) repeating steps 2, 3, and 4; and (10) collecting the ¹H signal in direct dimension; where X is selected from the group consisting of ¹⁵N and ¹³C.

In a specific example, the ¹H nucleus may be applied with 1.9 µs of the π/2 pulse and 3.8 µs of the π pulse.

In a specific example, the ¹⁵N nucleus may be applied with 9.2 µs of the π pulse.

In a specific example, the ¹³C nucleus may be applied with 7.0 µs of the π pulse.

In a specific example, the method may further include the following steps between steps (1) and (2): applying ¹H-¹⁵N cross-polarization (CP); applying a shape pulse on the ¹⁵N nucleus; suppressing a water signal with a MISSISIPPI pulse sequence; and applying ¹⁵N-¹H CP.

In a specific example, ¹H-¹⁵N CP may be applied at a contact time of 1 ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.

In a specific example, the shape pulse may be applied on the ¹⁵N nucleus at an excitation time of 625 µs.

In a specific example, ¹⁵N-¹H CP may be applied at a contact time of 300 µs, a ¹⁵N constant lock field at 50 kHz, and a ¹H constant lock field at 90 kHz.

In a specific example, the shape pulse may be selected from the group consisting of Q3, GAUSSI, SQUARE, SINC, RSONB, REBurp, CRP, and REBURP.

In the present disclosure, the method has the following advantages: (1) a proton-detected technology is adopted with a high sensitivity; (2) a ¹H-¹⁵N two-dimensional spectrum is constructed to observe correlation signals with a high accuracy; (3) the ¹H-¹⁵N TEDOR pulse parameter setting is simple without need to optimize according to the angle and the bond length of the hydrogen bond; (4) acquisition of the ¹H atoms is correlated to a chemical shift of two nitrogen atoms in the hydrogen bond, and the hydrogen bond can be directly observed at atomic level;(5) through the combination of selective pulses and TEDOR pulses, the overlapping of spectra is effectively reduced; and (6) in the N—H...N hydrogen bond, N—H ligated by chemical bonds has a distance of about 1.0 Å, and H and N atoms ligated by hydrogen bonds has a distance of about 1.9 Å. Therefore, with the ¹H-¹⁵N TEDOR experiment and by setting different mixing times, N—H groups ligated by chemical bonds, as well as H and N atoms ligated by hydrogen bonds, are detected for structural identification and characterization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of a two-dimensional ¹H-¹⁵N TEDOR pulse sequence; where Φ1 =x,x,x,x,-x,v,-x,-x; Φrec=x,-x,x, -x, -x, x, -x, x; rr/2 represents half a rotation period;

FIG. 1B shows a schematic diagram of a two-dimensional ¹H-¹³C TEDOR pulse sequence; where Φ1=x,x,x,x,-x, -x, -x, Φrec=x,-x,x, -x,-x,x,-x,x; rr/2 represents half a rotation period;

FIG. 1C shows a schematic diagram of a two-dimensional ¹H-¹⁵N-hNH(FSL)-[¹H,¹⁵N]-TEDOR pulse sequence; where Φ1=x,x,-x,-x; Φ2=y,y,y,y, -y.-y,-y,-y; Φrec=y,-y,-y,y,-y,y,y,-y; τr/2 represents half a rotation period;

FIG. 1D shows a schematic diagram of a two-dimensional ¹H-¹³C-hNH(FSL)-[¹H,¹³C]-TEDOR pulse sequence; where Φ1=x,x,-x,-x; Φ2=y,y,y,y, -y,-y,-y,-y; Φrec=y,-y,-y,y,-y,y,y,-y; rr/2 represents half a rotation period;

FIG. 2A shows a two-dimensional ¹H-¹⁵N TEDOR spectrum of a ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gel with a total evolution time of 100 µs (n=1);

FIG. 2B shows a two-dimensional ¹H-¹⁵N TEDOR spectrum of a ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gel with a total evolution time of 400 µs (n=4);

FIG. 2C shows a two-dimensional ¹H-¹⁵N TEDOR spectrum of a ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gel with a total evolution time of 600 µs (n=6);

FIG. 3A shows a two-dimensional ¹H-¹³C TEDOR spectrum of a ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gel with a total evolution time of 200 µs (n=2);

FIG. 3B shows a two-dimensional ¹H-¹³C TEDOR spectrum of a ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gel with a total evolution time of 600 µs (n=6);

FIG. 3C shows a schematic diagram of a G-C paired structure;

FIG. 4A shows a two-dimensional ¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR spectrum of a ¹³C,¹⁵N isotopes-unifonnly labeled (GGGGCC)₅RNA gel with a total evolution time of 200 µs (n=2);

FIG. 4B shows a two-dimensional ¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR spectrum of a ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gel with a total evolution time of 600 µs (n=6);

FIG. 4C shows a schematic diagram of a G-G paired structure;

FIG. 5A shows a two-dimensional ¹H-¹³C-hNH-(FSL)-[¹H,¹³C]-TEDOR spectrum of a mixed-labeled (GGGGCC)₅RNA gel with a total evolution time of 600 µs (n=6); and

FIG. 5B shows a schematic diagram of a G-G paired structure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure is further described below with reference to examples, which are not intended to limit the present disclosure. Various changes and modifications can be made to the present disclosure by those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present disclosure should be included within the protection scope of the claims of the present disclosure.

In order to realize the detection of intramolecular or intermolecular hydrogen bonds, the present disclosure provides a series of ssNMR methods for detecting N—H...N and N—H...O═C hydrogen bonds under a fast magic angle spinning (MAS). That is, two-dimensional (2D) proton detection ¹H-X(X=¹⁵N or ¹³C) transferred-echo double resonance (TEOOR), which is also known as ¹H-¹⁵NTEDOR and ¹H-¹³C TEDOR, as well as TEDOR-related experiments incorporating a selective pulse strategy (namely ¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR and ¹H-¹³C-hNH-(FSL)-[¹H,¹³C]-TEDOR) are used.

A key design of the TEDOR experiment includes: through two rotational echo double resonance (REDOR) modules, the transfer of ¹H to ¹⁵N or ¹³C and the transfer of ¹³N or ¹³C to ¹H are achieved to establish a ¹H-¹⁵N or ¹H-¹³C two-dimensional chemical shift-correlated spectrum, so as to obtain correlation information between the H atoms and atoms on both sides of the hydrogen bond, thereby directly observing the hydrogen bond. REDOR is a pulse sequence of heteronuclear dipole recoupling under MAS conditions; under the MAS conditions, the dipole coupling effect of phase-coupled spin pairs is averaged by the MAS. In the evolution period of a detection nucleus (such as ¹H), a π pulse is applied to a channel of the coupling nucleus (X, X=¹⁵N or ¹³C) every half of rotation period, such that the heteronuclear dipole coupling effect is not averaged by the MAS, and the single quantum coherence of ¹H (¹Hx) evolves into a ¹H-X multi-quantum coherence (¹HyXz). The optimal evolution time is related to a distance between the ¹H and X atoms. By adjusting the total evolution time, a hydrogen atom and the X atoms on both sides of the hydrogen atom (X=¹⁵N or ¹³C) can be detected separately. After recording the chemical shift of X, ¹HyXz is transformed into ¹Hx by a REDOR module, and direct dimension sampling is conducted on ¹H.

In addition to 2D¹H-X TEDOR, in the present disclosure, a selective pulse is combined with the TEDOR for the first time. Selective pulses can filter out specific signals through spectral editing. In the present disclosure, a spectral editing method includes a cross-polarization (CP) experiment selected based on a contact time in a CP process, and a pulse sequence selected based on the frequency.

By combining the CP pulse, frequency selective pulses (FSLs) and TEDOR block, ¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR and ¹H-¹³C-hNH(FSL)-[¹H,¹³C]-TEDOR pulses. The magnetic resonance signals are transferred by ¹H-¹⁵N CP in an hNH pulse; the frequency of ¹⁵N is selected, and signals of a specific frequency are only retained, so as to select the ¹⁵N signals with a specific chemical shift; furthermore, the ¹⁵N signals with a specific chemical shift are transmitted to chemically-bonded ¹H signals through the ¹H-¹⁵N CP of short contact time. Therefore, only imino or amino signals of specific chemical shifts are retained by an hNH(FSL) pulsing module. Finally, the ¹R-¹⁵N or ¹H-¹³C two-dimensional spectrum is constructed by ¹H-X TEDOR to realize specific detection of the N—H ... N and N—H...O═C hydrogen bonds. Compared with ¹H-X TEDOR, these two pulses can realize the specific detection of hydrogen bonds involving certain imino and amino groups (such as the imino groups of guanine and uridine, and the amino groups of guanine, cytosine, and adenine), which can effectively avoid interference of other signals to further improve an accuracy of hydrogen bond detection.

In addition, in the present disclosure, by using the fast MAS condition (40 kHz), a transverse relaxation time of ¹H is significantly longer than that of medium- and low-speed MAS condition, which effectively reduces the signal loss during ¹H-X evolution, thereby improving a sensitivity of the experiment.

The present disclosure is achieved by the following technical solutions:

A ssNMR method for detecting a hydrogen bond structure includes the following steps:

(1) A sample to be tested is prepared, including a ¹⁵N or ¹³C label.

(2) A pulse sequence is set, including the following steps:

For the 2D¹H-X TEDOR pulse (X=¹⁵N or ¹³C), the following operations are conducted: a ¹H nucleus is excited with a π/2 pulse, and then a π pulse is applied to an X nucleus every half rotation period for a certain number of π pulses; the π pulse is applied in a ¹H channel to suppress the ¹H chemical shift anisotropy and the ¹H chemical shift evolution of ¹H during the ¹H-X evolution: the π pulse is repeatedly applied on the X channel every half rotation period, with the number equal to that of the π pulse on the X channel in front of the ¹H π pulse; after the above step, the ¹H channel is locked to suppress the solvent signal; a 90° pulse is applied on the ¹H and X atoms, respectively, and a chemical shift (t1) is recorded in an X dimension, and a 90° pulse is applied on the ¹H and X atoms, respectively, after the recording is completed. After completing the above operations for the two groups of REDOR pulse sequences, the ¹H signal is collected, and the ¹H-X two-dimensional spectrum is constructed.

For the ¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR pulse and ¹³H-¹³C-hNH-(FSL)-[¹H,¹³C]-TEDOR pulse, the following operations are conducted: the ¹H nucleus is excited with the x/2 pulse, ¹H-¹⁵N CP realizes the polarization of ¹⁵N nucleus with a low sensitivity, and a contact time is set to 1 ms to 10 ms; with a shape pulse, the ¹⁵N off-set and a pulse length are adjusted, a ¹⁵N signal with specific chemical shift is selected; the ¹⁵N signal is transferred to a ¹H linked by a H—N chemical bond, and a contact time is set to 100 µs to 1,000 µs. The subsequent ¹H-X TEDOR module design is consistent with the other experiments described above.

(3) The sample is detected using an ssNMR spectrometer to obtain a two-dimensional spectrum of the sample.

(4) According to the two-dimensional spectrum of the sample, signals in the spectra at different mixing times are compared to determine whether there is a NH...N or N—H...O═C hydrogen bond.

The sample is a biological macromolecule or other substances containing hydrogen bonds, where the biological macromolecule includes DNAs, RNAs, and nucleic acid-protein complexes; and the other substances containing hydrogen bonds from N—H...N or N—H...O═C include, but are not limited to, N-containing soft substances, biological materials, and polymer materials.

The sample includes a solid sample, a gel sample or a sedimentation soluble sample.

Preparation methods of a RNA sample include in vitro transcription, solid-phase synthesis, or biological extraction.

When preparing the RNA sample, the RNA sample is purified by polyacrylamide gel electrophoresis.

A water signal is suppressed by pulsing on ¹H to suppress the solvent signals.

In ¹H-¹⁵N-hNH(FSL)-[¹H,¹⁵N]-TEDOR and ¹H-¹³C-hNH-(FSL)-[¹H,¹³C]-TEDOR pulses, the shape pulses used is selected from the group consisting of Q3, GAUSSI, SQUARE, SINC, RSONB, REBurp, CRP, and REBURP.

Example 1

In this example, the pulse sequence was tested using an RNA gel formed by (GGGGCC)₅ sequence as a model. The GGGGCC sequence was part of a C9ORF72 gene, with repeat expansion associated with various neurodegenerative diseases. Studies had shown that repeat-expanded GGGGCC RNA might form RNA gels under specific conditions, and its structure had not been determined. The RNA gel formed by (GGGGCC)₅sequence was analyzed for the first time using ssNMR methods.

1. Preparation of RNA Samples (1) Construction of DNA Template

In this example, (GGGGCC)₅RNA was obtained by an in vitro transcription method, and a corresponding DNA template was designed according to the target RNA. The DNA template included two complementary DNA single strands, namely a forward strand (R strand) and a reverse strand (F strand), and sequences were as follows:

 R strand: 5′-GGCCCCGGCCCCGGCCCCGGCCCCGGCCCCTATAGTGAGTCGTATTAA-3′

 F strand: 5′-TTAATACGACTCACTATAGGGGCCGGGGCCGGGGCCGGGGCCGGGGCC-3′

The single-stranded DNA was centrifuged, dissolved, and annealed by heating to form a linear double-stranded DNA template (with a final concentration of 25 µM).

(2) RNA Samples Preparation by in Vitro Transcription

In the present disclosure, two RNA samples were prepared, namely ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA and mixed labeled (GGGGCC)₅RNA prepared by adding ¹³C-labeled GTP and ¹⁵N-labeled GTP separately.

¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA samples were prepared by 5 mL of an in vitro transcription system: 2,600 µL of DEPC-treated water, 665 µL of 100 mM¹³C¹⁵N isotope-labeled GTP, 335 pC. of 100 mM¹³C¹⁵N isotope-labeled CTP, 500 µL of Tris buffer (containing 400 mM Tris/HCl, pH 8.0, 10 mM Spermidine, 0.1% Triton X-100), 50 µL of 1 M dithiothreitol (DTT), 200 µL of 1 M MgCl₂, 150 µL of 25 µM DNA template, and 500 µL of 2 mg/mL T7 RNA polymerase were sequentially added to a 50 mL centrifuge tube. Incubation was conducted in a 37° C. water bath for 24 h to ensure complete reaction of the starting materials.

Mixed-labeled (GGGGCC)₅RNA was prepared by 5 mL of an in vitro transcription system: 2,600 µL of DEPC-treated water, 330 µL of 100 mM¹³C isotope-labeled GTP, 330 µL of 100 mM ¹⁵N isotope-labeled GTP, 340 µL of non-isotopo-labeled CTP, 500 µL of Tris buffer (containing 400 mM TrisiHCl, pH 8.0, 10 mM Spermidine, 0.1% Triton X-100), 50 µL of 1 M dithiothreitol (DTT), 200 µL of 1 M MgCb, 150 µL of 25 uM DNA template, and 500 µL of 2 mg/mL ‘17 RNA polymerase were sequentially added to a 50 mL centrifuge tube. Incubation was conducted in a 37° C. water bath for 24 h to ensure complete reaction of the starting materials. GTP in the resulting mixed-labeled (GGGGCC)₅RNA was either ¹³C-labeled or ¹⁵N-labeled only.

(3) RNA Purification

After the transcription reaction was completed, the RNA was purified by 12% polyacrylamide gel electrophoresis (PAGE), and a target band was obtained by cutting a gel under UV light after the electrophoresis. The band containing the target RNA was soaked in an extraction buffer (20 mM Tris-HCl, 300 mM sodium acetate, and 1 mM EDTA, pH 7.4) at 37° C. for about 12 h, and a solution was collected.

(4) Preparation of RNA Gel Samples

A (GGGGCC)₅RNA sample was dissolved in a solution containing 5 mM Tris (pH=7.0) and 10 mM MgCl₂ by ultrafiltration, heated at 95° C. for 3 min, and then cooled down to 37° C. in a gradient manner at 3° C./min, to obtain the RNA gel samples.

2. Pulse Sequence Setup and Method for Detecting Hydrogen Bond

2D¹H-¹⁵N TEDOR, 2D¹H-¹³C TEDOR, 2D¹H-¹⁵N-hNH-(FSL)-TEDOR, and 2D¹H-¹³C-hNH-(FSL)-[¹H,¹³C]-TEDOR experiments each were completed at a MAS of 40 kHz, and one rotation period was 25 µs, and half a rotation period was 12.5 µs. In this example, π/2 had the same definition as 90°, and π had the same definition as 180°. In this example, an ssNMR instrument was used.

The 2D¹H-¹⁵N TEDOR pulse sequence (FIG. 1A) was set up as follows:

-   a¹H nucleus of the RNA sample was excited with a π/2 pulse, and -   2 π pulses were applied to a ¹⁵N nucleus of the RNA sample every     half rotation period; -   ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H     during ¹H-¹⁵N evolution were suppressed by applying a π pulse to the     ¹H nucleus of the RNA sample; -   2 π pulses were applied to the ^(I5)N nucleus of the RNA sample     every half rotation period; -   a 90° pulse was separately applied on the ¹H and ¹⁵N nuclei of the     RNA sample,

The ¹⁵N chemical shift was recorded in the indirect dimension

The 90° pulse was separately applied on the ¹H and ¹⁵N nuclei of the RNA sample.

REDOR pulse sequence: a total of 2 π pulses were applied to the ¹⁵N nucleus every half rotation period; the π pulse was applied on the ¹H nucleus to suppress ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H during ¹H-¹⁵N evolution; a total of 2 π pulses were applied on the ¹⁵N nucleus every half rotation period;

After completing the above operations for the two groups of REDOR pulse sequences, the ¹H signal was collected, and the ¹H-¹⁵N two-dimensional spectrum was constructed.

Referring to FIG. 1A, the number of π pulses was controlled by adjusting a value of n in the experiment. When n=1, a total of 8 π pulses were applied to the ¹⁵N nucleus, with a total evolution time of 100 µs. When n was an integer greater than 1, a total of 8n π pulses were applied to the ¹⁵N nucleus, with a total evolution time of 100 n µs.

In this experiment, the direct dimension (F1: ¹H) and the indirect dimension (F2: ¹⁵N) took 958 points and 128 points in t1 evolution period and t2 acquisition period, respectively.

The π/2 pulse applied to the ¹H nucleus was 1.9 µs, and the π pulse applied to the ¹H nucleus was 3.8 µs, and

The π/2 pulse applied on the ¹⁵N nucleus was 4.6 µs, and the π pulse applied on the ¹⁵N nucleus was 9.2 µs.

A recycle waiting time was 2 s.

The 2D¹H-¹³C TEDOR pulse sequence (FIG. 1B) was set up as follows:

-   a¹H nucleus of the RNA sample was excited with a π/2 pulse, and -   2 π pulses were applied to a ¹³C nucleus of the RNA sample every     half rotation period; -   ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H     during ¹H-¹³C evolution were suppressed by applying a π pulse to the     ¹H nucleus of the RNA sample; -   2 π pulses were applied to the ¹³C nucleus of the RNA sample every     half rotation period; -   a 90° pulse was separately applied on the ¹H and ¹³C nuclei of the     RNA sample, -   a chemical shift t1 was recorded in a ¹³C dimension, and -   the 90° pulse was separately applied on the ¹H and ¹³C nuclei of the     RNA sample.

REDOR pulse sequence: a total of 2 π pulses were applied to the ¹³C nucleus of the RNA sample every half rotation period; the π pulse was applied on the ¹H nucleus of the RNA sample to suppress ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H during ¹H-¹³C evolution; a total of 2 π pulses were applied on the ¹³C nucleus every half rotation period;

After completing the above operations for the two groups of REDOR pulse sequences, the ¹H signal was collected, and the ¹H-¹³C two-dimensional spectrum was constructed.

Referring to FIG. 1B, the number of π pulses was controlled by adjusting a value of n in the disclosure experiment. When n=1, a total of 8 π pulses were applied to the ¹³C nucleus, with a total evolution time of 100 µs. When n was an integer greater than 1, a total of 8n π pulses were applied to the ¹³C nucleus, with a total evolution time of 100 n µs.

In this experiment, the direct dimension (F1: ¹H) and the indirect dimension (F2: ¹³C) took 958 points and 128 points in t1 evolution period and t2 acquisition period, respectively.

The π/2 pulse applied to the ¹H nucleus was 1.9 µs, and the π pulse applied to the ¹H nucleus was 3.8 µs, and

The π/2 pulse applied on the ¹³C nucleus was 3.5 µs, and the _(π) pulse applied on the ¹³C nucleus was 7.0 µs.

A recycle waiting time was 2 s.

The 2D¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR pulse sequence (FIG. 1C) was set up as follows:

-   a¹H nucleus of the RNA sample was excited with a π/2 pulse, and -   In a specific example, ¹H-¹⁵N CP was applied at a contact time of 1     ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field     lock at 90 kHz; -   a shape pulse was applied to the ¹⁵N nucleus at an excitation center     of 90 ppm and a excitation time of 625 µs, and a ¹⁵N signal at a     specific chemical shift was selected; -   a water signal was suppressed with a MISSISIPPI pulse sequence; and -   ¹⁵N-¹H CP was applied at a contact time of 300 µs, a ¹⁵N constant     field lock at 50 kHz, and a ¹H constant field lock at 90 kHz, and     the ¹⁵N signal was transferred to the ¹⁵N-chemically bonded ¹H     nucleus.

2 π pulses were applied to a ¹⁵N nucleus of the RNA sample every half rotation period;

-   ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H     during ¹H-¹⁵N evolution were suppressed by applying a π pulse to the     ¹H nucleus; -   2 π pulses were applied to the ¹⁵N nucleus every half rotation     period; -   a 90° pulse was separately applied on the ¹H and ¹⁵N nuclei, and the     ¹⁵N chemical shift was recorded in the indirect dimension, and -   the 90° pulse was separately applied on the ¹H and ¹⁵N nuclei.

REDOR pulse sequence: a total of 2 π pulses were applied to the ¹⁵N nucleus every half rotation period; the π pulse was applied on the ¹H nucleus to suppress ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H during ¹H-¹⁵N evolution; a total of 2 π pulses were applied on the ¹⁵N nucleus every half rotation period;

After completing the above operations for the two groups of REDOR pulse sequences, the ¹H signal was collected, and the ¹H-¹⁵N two-dimensional spectrum was constructed.

The number of π pulses was controlled by adjusting a value of n in the disclosure experiment. When n=1, a total of 8 π pulses were applied to the ¹⁵N nucleus, with a total evolution time of 100 µs. When n was an integer greater than 1, a total of 8n π pulses were applied to the ¹⁵N nucleus, with a total evolution time of 100 n µs.

In this experiment, the direct dimension (F1: ¹H) and the indirect dimension (F2: ¹⁵N) took 958 points and 80 points in t1 evolution period and t2 acquisition period, respectively.

The π/2 pulse applied to the ¹H nucleus was 1.9 µs, and the π pulse applied to the ¹H nucleus was 3.8 µs, and

The π/2 pulse applied on the ¹⁵N nucleus was 4.6 µs, and the π pulse applied on the ¹⁵N nucleus was 9.2 µs.

A recycle waiting time was 2 s.

The shape pulse was Gaus1_l80i.1000.

The 2D¹H-¹³C-hNH-(FSL)-(¹H,¹³C]-TEDOR pulse sequence (FIG. 1D) was set up as follows:

-   a¹H nucleus of the RNA sample was excited with a π/2 pulse, and -   In a specific example, ¹H-¹⁵N CP was applied at a contact time of 1     ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field     lock at 90 kHz. -   a shape pulse was applied to the ¹⁵N nucleus at an excitation center     of 150 ppm and a excitation time of 625 µs, and a ¹⁵N signal at a     specific chemical shift was selected; -   a water signal was suppressed with a MISSIS1PPI pulse sequence; and -   ¹⁵N-¹H CP was applied at a contact time of 300 µs, a ¹⁵N constant     field lock at 50 kHz, and a ¹H constant field lock at 90 kHz, and     the ¹⁵N signal was transferred to the ¹⁵N-chemically bonded ¹H     nucleus.

2 π pulses were applied to a ¹³C nucleus of the RNA sample every half rotation period;

-   ¹H chemical shift anisotropy and ¹H chemical shift evolution of ¹H     during ¹H-¹³C evolution were suppressed by applying a π pulse to the     ¹H nucleus of the RNA sample; -   2 π pulses were applied to the ¹³C nucleus of the RNA sample every     half rotation period; -   After completion, the hydrogen was locked for 500 µs. The ¹H     constant field lock was 130 kHz, suppressing the solvent signal; -   a 90° pulse was separately applied on the ¹H and ¹³C nuclei, -   The ¹³C chemical shift was recorded in the indirect¹³C dimension,     and -   the 90° pulse was separately applied on the ¹H and ¹³C nuclei of the     RNA sample. -   REDOR pulse sequence: a total of 2 π pulses were applied to the ¹³C     nucleus every half rotation period; -   ¹H chemical shift anisotropy and ¹H chemical shift evolution     during¹³C evolution were suppressed by applying a π pulse to the ¹H     nucleus; -   A total of 2 π pulses were applied to the ¹³C nucleus every half     rotation period; -   After completing the above operations for the two groups of REDOR     pulse sequences, the ¹H signal was collected, and the ¹H-¹³C     two-dimensional spectrum was constructed.

The number of π pulses was controlled by adjusting a value of n in the disclosure experiment. When n=1, a total of 8 π pulses were applied to the ¹³C nucleus, with a total evolution time of 100 µs. When n was an integer greater than l, a total of 8n π pulses were applied to the ¹³C nucleus, with a total evolution time of 100 n µs

In this experiment, the direct dimension (F1: ¹H) and the indirect dimension (F2: ¹³C) took 958 points and 128 points in t1 evolution period and t2 acquisition period, respectively.

The π/2 pulse applied to the ¹H nucleus was 1.9 µs, and the π pulse applied to the ¹H nucleus was 3.8 µs, and

The π/2 pulse applied on the ¹⁵N nucleus was 4.6 µs, and the π pulse applied on the ¹⁵N nucleus was 9.2 µs.

The π/2 pulse applied on the ¹³C nucleus was 3.5 µs, and the π pulse applied on the ¹³C nucleus was 7.0 µs.

A recycle waiting time was 2 s.

The shape pulse was GausI_180i. 1000,

3. Spectral collection and result analysis

In the present disclosure, two-dimensional ¹H-¹⁵N TEDOR spectra of ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gels were collected (FIG. 2A to FIG. 2C). The results were as follows: at a total evolution time of 100 µs (short mixing time), only signal peaks with ¹H/¹⁵N chemical shifts of 13 ppm/147.6 ppm and 10.6 ppm/142.0 ppm were observed, representing the N1-H1 cross peak of a guanine base, namely the intra guanine base NH signal. When the total evolution time was 400 µs, in addition to the internal cross peaks of the bases, the related information between the bases was observed, namely the signal peaks with ¹H/¹⁵N chemical shifts of 13 ppm/197 ppm, representing a cross peak of H1 in guanine base and the N3 in cytosine base, and proving the existence of N—H...N hydrogen bond between G-C base pair. In addition, when the total evolution time was extended to 600 µs (short mixing time), under the influence of relaxation, the intra guanine base NH signal disappeared; while long-range signals (correlated signals between the guanine and the cytosine bases) could still be observed. Therefore, the NH...N hydrogen bond in RNA gel systems could be characterized using ¹H-¹⁵N TEDOR experiments.

In the present disclosure, two-dimensional ¹H-¹³C TEDOR spectra of ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gels were collected (FIG. 3A to FIG. 3B). The results were as follows: at a total evolution time of 300 µs (short mixing time), only signal peaks with ¹H/¹³C chemical shifts of 13 ppm/156 ppm and 13 ppm/161 ppm were observed, representing the C2—H1 and C6-H 1 cross peaks, both being the intra guanine base CH signals. When the total evolution time was 700 µs, in addition to the internal cross peaks of the bases, the related information between the bases was observed, namely the signal peaks with ¹H/¹³C chemical shifts of 13 ppm/168 ppm, representing a cross peak of H1 in guanine base and C4 in cytosine base, and proving the existence of N—H...N hydrogen bond between G-C (FIG. 3C). Therefore, the hydrogen bond in RNA gel systems could be characterized using ¹H-¹³C TEDOR experiments.

In addition, in the present disclosure, two-dimensional 2D¹H-¹⁵N-hNH-(FSL)-[¹H,¹⁵N]-TEDOR spectra of ¹³C,¹⁵N isotopes-uniformly labeled (GGGGCC)₅RNA gels were collected (FIG. 4A to FIG. 4B). The results were as follows: at a total evolution time of 200 µs (short mixing time), only signal peaks with ¹H/¹⁵N chemical shifts of 9 ppm/80 ppm and 8 ppm/80 ppm were observed, representing a N2-H2 cross peak, namely the intra guanine base NH₂ signal. When the total evolution time was 600 µs, in addition to the internal cross peaks of the bases, the related information between bases was observed, namely the signal peaks with ¹H/¹⁵N chemical shifts of 9 ppm/232.7 ppm, representing a cross peak of H2 in G base and N7 paired to the guanine base, and demonstrating the existence of the N—H...N hydrogen bond between the G-G Hoogesteen base pairings (FIG. 4C).

In the present disclosure, two-dimensional 2D¹H-¹³C-hNH-(FSL)-[¹H,¹³C]-TEDOR spectra of (GGGGCC)₅RNA gels were collected (FIG. 5A). The results were as follows: when the total evolution time was 600 µs (long mixing time), signal peaks with ¹H/¹³C chemical shifts of 10.78 ppm/160.9 ppm were observed, representing a cross peak of C6 in guanine base and H1 paired to the guanine base, and demonstrating the existence of N—H ... O═C hydrogen bond between G-G pairings (FIG. 5B).

Compared with the prior art, the present disclosure has the following beneficial effects:

In the present disclosure, a ¹H-X TEDOR pulse sequence is designed by combining proton-detected technology with transfer echo double resonance pulse technology, and a ssNMR method is established for specific detection of N—H...N or N—H...O═C hydrogen bond. TEDOR achieves coherent transfer through a heteronuclear recoupling method based on dipole coupling, and establishes ¹H-X chemical shift correlations. In this experiment, in the ¹H-XTEDOR method, after the excitation of ¹H, π pulses are applied to the X channel every half rotation period. After several rotation cycles, dipole recoupling is achieved and the ¹H magnetization convert to multi-quantum ¹H-X coherence. The coupled ¹H-X coherence is converted into a detectable ¹H coherence through the second REDOR period, and the ¹H chemical shift is recorded to complete the two-dimensional experiment. In the TEDOR method, the cross-peak intensity is related to the distance between two nuclei and the total mixing time of REDOR period. Through the TEDOR pulse experiment, the chemical shift correlation can be realized between ¹H and the atoms on both sides in the N—H...N or N—H...O═C hydrogen bond, meanwhile, by adjusting the total time, the chemical shift correlation between the covalently bound N and H atoms, as well as the chemical shift correlation between X and H atoms connected by hydrogen bond can be identified.

In addition, in the present disclosure, the selective pulses, including the CP pulse and the frequency selective pulse, are combined with the TEDOR pulse for the first time, thus effectively reducing the spectral overlapping.

In the present disclosure, the method has the following advantages: (1) a proton detected technology is adopted with a high sensitivity; (2) a ¹H-¹⁵N two-dimensional spectrum is constructed to observe relevant signals with a high accuracy; (3) TEDOR pulse parameter setting is simple, without need to be optimized according to the angle and the bond length of the hydrogen bond; (4) acquisition of the ¹H atoms is related to a chemical shift of two nitrogen atoms in the hydrogen bond, and the hydrogen bond can be directly observed at atomic level; (5) through the combination of selective pulses and TEDOR pulses, the overlapping of spectra is effectively reduced; and (6) in the N—H...N hydrogen bond, N-H connected by chemical bonds has a distance of about 1.0 Å, and H and N atoms connected by hydrogen bonds has a distance of about 1.9 Å. Therefore, with the ¹H-¹⁵N TEDOR experiment, by setting different mixing times, N-H groups connected by chemical bonds, as well as H and N atoms connected by hydrogen bonds, are detected for structural identification and characterization. 

1. A solid-state nuclear magnetic resonance (ssNMR) method for detecting a hydrogen bond structure, comprising the following steps: (1) exciting a ¹H nucleus of an RNA sample with a π/2 pulse; (2) applying two π pulses every half rotation period on an X nucleus of the RNA sample; (3) applying a π pulse on the ¹H nucleus of the RNA sample; (4) applying two π pulses every half rotation period on the X nucleus of the RNA sample; (5) applying a 90° pulse on ¹H and X atoms of the RNA sample; (6) recording a chemical shift of X in indirect dimension; (7) applying the 90° pulse on the ¹H and X nuclei of the RNA sample; (8) repeating steps 2, 3, and 4; and (9) collecting the ¹H signal in direct dimension; wherein X is selected from the group consisting of ¹⁵N and ¹³C.
 2. The ssNMR method for detecting a hydrogen bond structure according to claim 1, wherein the ¹H nucleus is applied with 1.9 µs of the π/2 pulse and 3.8 µs of the π pulse.
 3. The ssNMR method for detecting a hydrogen bond structure according to claim 1, wherein the ¹⁵N nucleus is applied with 9.2 µs of the π pulse.
 4. The ssNMR method for detecting a hydrogen bond structure according to claim 1, wherein the ¹³C nucleus is applied with 7.0 µs of the π pulse.
 5. The ssNMR method for detecting a hydrogen bond structure according to claim 1, wherein the method further comprises the following steps between steps (1) and (2): applying¹H-¹⁵N cross-polarization (CP); applying a shape pulse on the ¹⁵N nucleus; suppressing a water signal with a MISSISIPPI pulse sequence; and applying⁵⁵N-¹H CP.
 6. The ssNMR method for detecting a hydrogen bond structure according to claim 5, wherein ¹H-¹⁵N CP is applied at a contact time of 1 ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.
 7. The ssNMR method for detecting a hydrogen bond structure according to claim 5, wherein the shape pulse is applied on the ¹⁵N nucleus at an excitation time of 625 µs.
 8. The ssNMR method for detecting a hydrogen bond structure according to claim 5, wherein ¹⁵N-¹H CP is applied at a contact time of 300 µs, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.
 9. The ssNMR method for detecting a hydrogen bond structure according to claim 5, wherein the shape pulse is selected from the group consisting of Q3, GAUSSI, SQUARE, SINC, RSONB, REBurp, CRP, and REBURP.
 10. The ssNMR method for detecting a hydrogen bond structure according to claim 2, wherein the method further comprises the following steps between steps (1) and (2): applying¹H-¹⁵N CP; applying a shape pulse on the ¹⁵N nucleus; suppressing a water signal with a MISSISIPPI pulse sequence; and applyíng¹⁵N-¹H CP.
 11. The ssNMR method for detecting a hydrogen bond structure according to claim 3, wherein the method further comprises the following steps between steps (1) and (2): applying¹H-¹⁵N CP; applying a shape pulse on the ¹⁵N nucleus; suppressing a water signal with a MISSISIPPI pulse sequence; and applying¹⁵N-¹H CP.
 12. The ssNMR method for detecting a hydrogen bond structure according to claim 4, wherein the method further comprises the following steps between steps (1) and (2): applying¹H-¹⁵N CP; applying a shape pulse on the ¹⁵N nucleus; suppressing a water signal with a MISSISIPPI pulse sequence; and applying¹⁵N-¹H CP.
 13. The ssNMR method for detecting a hydrogen bond structure according to claim 10, wherein ¹H-¹⁵N CP is applied at a contact time of 1 ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.
 14. The ssNMR method for detecting a hydrogen bond structure according to claim 11, wherein ¹H-¹⁵N CP is applied at a contact time of 1 ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.
 15. The ssNMR method for detecting a hydrogen bond structure according to claim 12, wherein ¹H-¹⁵N CP is applied at a contact time of 1 ms, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.
 16. The ssNMR method for detecting a hydrogen bond structure according to claim 10, wherein the shape pulse is applied on the ¹⁵N nucleus at an excitation time of 625 µs.
 17. The ssNMR method for detecting a hydrogen bond structure according to claim 11, wherein the shape pulse is applied on the ¹⁵N nucleus at an excitation time of 625 µs.
 18. The ssNMR method for detecting a hydrogen bond structure according to claim 12, wherein the shape pulse is applied on the ¹⁵N nucleus at an excitation time of 625 µs.
 19. The ssNMR method for detecting a hydrogen bond structure according to claim 10, wherein ¹⁵N-¹H CP is applied at a contact time of 300 µs, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz.
 20. The ssNMR method for detecting a hydrogen bond structure according to claim 11, wherein ¹⁵N-¹H CP is applied at a contact time of 300 µs, a ¹⁵N constant field lock at 50 kHz, and a ¹H constant field lock at 90 kHz. 